Investigations of the Counter-Intuitively Short Lifetimes of the Visible Emission from Laser-Induced Plasma from RDX

Abstract

Abstract

Through multiple studies on laser generated plasmas of materials, four phenomena have been curious and unexplainable regarding the non-energetic material polycarbonate (PC) and energetic material RDX (Cyclotrimethylenetrinitramine). First, the broad, blackbody emission of laser-induced PC plasmas decreases in intensity but does not shift in wavelength through the vast majority of the emission process.1 Second, where the emission from the PC lasts for hundreds of microseconds, the emission from RDX is extremely short lived (ns). Third, laser induced ablation pits on RDX (ablation is the clean removal of material from a surface) are ten times as deep as they are with PC.1,2 Fourth, the nature of the power density threshold for laser ignition that has been observed for RDX remained wholly unknown. To address the latter two issues, a model study was carried out to investigate whether the laser ignition of RDX at high laser fluence is simply the result of thermal ignition resulting from heating by a blackbody plasma raising the temperature of the RDX above the thermal threshold for reaction. Using a simple model of a plasma plume as a blackbody above the solid RDX surface, pit temperatures were estimated and the volume of RDX heated above the sublimation temperature and compared to the pit sizes observed in prior laser ablation experiments. The observation that the simple model, using only a few reasonable assumptions, produces laser ablation pits in RDX that are approximately ten times deeper than those observed on PC serves as strong evidence that the deeper pits are the result of black body plasma re-radiated onto the RDX surface. Further increasing the laser power in the model to a value comparable to those at the reported threshold for laser ignition results in temperatures of over 900K at the surface which would result in thermal ignition. The lack of a wavelength shift as the signal level drops seems to indicate that the plasma is emitting at a constant temperature; however, that makes no sense as the plasma is losing energy through emission of light, so it should expected to cool. To explain this observation, Casper hypothesized that the plasma in laser ablation is optically dense, meaning that only the surface is visible to the observer, and that the emission at constant wavelength, or constant temperature, is the result of a phase transition from a strongly emitting plasma to much less emissive neutral gas at the surface. Within this model, the decrease in intensity indicates a decrease in surface area of the plasma as the interface recedes radially inward, as the plasma radiatively cools. An improved experimental setup using a fast camera to image plumes directly as well as spectrally using a diffraction grating spectrometer was built. The setup is identical to Casper’s configuration, except that the slow camera and pulsed image intensifier, which could only collect one spectrum per laser pulse was replaced with a fast camera. In the new configuration, used for all experiments in this work, the fast camera was used to collect 20 images, each 100 µs apart in time, following each laser pulse. Collecting many more spectra, it became possible to construct an average RDX spectrum revealing that RDX plasmas are much hotter than PC plasmas, with a surface temperature estimated at 8725K +/- 50, compared to 2240K +/- 5 for PC. As a result, the rapid disappearance of the visible RDX emission is proposed to result from the much hotter surface of the RDX plasma radiating away its energy much faster than the lower temperature PC plasma. The results of this work are strong evidence in support of the proposition that these phenomena are the result of a plasma/gas phase transition.

Collection of hundreds of images of laser ablation plumes also revealed a new anomaly. When the same region of RDX coated PC is exposed to a series of laser shots, visible emission from the first shot is bright, then goes dark for the second and third shots, and returns to bright again for the subsequent shots. It is proposed that pure RDX which is cleanly ablated results in the bright image of the first images of the first and often the second shots. White layers that cannot be removed with solvent are observed on the PC around the laser irradiation pit following laser ablation. They are attributed to subsurface melting of the PC, into which RDX can diffuse. These RDX+PC mixtures are described as “snow-pack” in this work. By the second or third laser shot, the top layer of RDX has been removed, and the RDX+PC mixture is being ablated and the plasma appears dark. This is consistent with previous reports that RDX needs PC to initiate via laser irradiation under the conditions used in these experiments, and that without the PC component, the pure RDX plasma does not ignite. After the RDX and “snow-pack” have been fully removed, the images of the plasmas return to bright. The RDX and underlying “snow-pack” removal can take several laser shots for plasmas to return to bright, dependent on the thickness of the RDX film, and how much RDX diffused into the molten PC to create “snow-pack.” It was also found that audio recordings on a smart phone indicate louder pops associated with the dark plasmas (energetic), which could provide an inexpensive way to investigate laser ignition further. Analysis of a histogram of plume photos indicates a distribution of pixel intensities that does not change in intensity or shift through time but just decreases in population. This observation indicates that the plasma emits at a constant temperature, and that proceeds by a reduction of the number of hot pixels, rather than a drop in their intensity. Finally, out of curiosity, fast spectra were collected for the laser ablation of a few metals on double sided tape. Spectra of the metals on tape show signs of the plain tape, PC, and features not common to either, making blackbody fitting difficult. Only Iron could be fit to a blackbody, with a best fit temperature of 12,450K +/- 225, compared to an estimated 7920K +/-15 temperature for plain double sided tape.

REFERENCES

1. Casper IV, Walter F. A microsecond time-resolved spectroscopic study of laser induced plasmas and their interactions with solid materials. Ph.D. Dissertation, Auburn University, Auburn, AL, 2015. 2. J.L. Gottfried, Influence of exothermic chemical reactions on laser-induced shock waves, Phys. Chem. Chem. Phys. 16, 21452 (2014)